1. Field of the Invention
The present invention relates to fabric stretchers and more particularly to an apparatus for tensioning and re-tensioning screen fabric for use in screen printing processing or the like.
2. Description of the Prior Art
Stretching frames are used for a wide variety of applications, including oil painting and the graphic arts. In the graphic arts, screen printing frames are used in the screen printing industry in association with a fabric panel of particular mesh size. The fabric panel as used in the screen print industry is manufactured from polyester and is typically supported and positively attached directly to the frame, then tensioned between opposing sides of the screen printing frame. The term “printing screen” as used in the industry refers to a screen printing frame with fabric panel secured thereto, either stenciled or as yet unstenciled. The mesh, when uniformly stretched in a taut plane, provides the foundation for a stencil. Printing ink is then forced through the open areas of the stencil by means of a blade or squeegee creating an image in the shape of the stencil on the work piece positioned beneath the fabric panel. In order to achieve optimum results, several factors must be considered. The fabric panel must be maintained at the recommended working tension throughout the print run so the pattern is transferred with accuracy. Tension variation must be kept to a minimum. Off contact must be kept to a minimum.
During the printing stroke, the squeegee blade deflects the fabric panel downwardly forcing it into direct contact with the work piece during its passage across the screen. This deflection causes the fabric panel to be stretched from its undisturbed condition. The constant cycle of stretching and snapping back accelerates the deterioration of both the fabric panel and the stencil, especially if the fabric panel is skewed during installation. Proper tensioning decreases off contact, improves image quality and provides longer stencil and image life. Additionally, as the fabric panel undergoes these distorting forces during the print stroke, the stencil in the screen may also be distorted. This can result in a slightly distorted image on the work piece.
Typical tubular alloy screen printing frames used to stretch the fabric panel utilize tensioning devices positioned in most cases on all four sides of the frame. The fabric panel is pulled over these tensioning devices and locked in place without the necessary squaring means. In doing so, the screen printing frame may become slightly distorted and this distortion can cause the off contact distance between the screen and the work piece to vary across the print area with a consequent reduction in print quality.
The essence of re-tensioning is to maintain a specific tension value in newtons. The majority of screen printing frames currently available do not allow the fabric panel and stencil to be easily removed and reinstalled on the frame, nor do they allow for easy retensioning of the fabric panel during operation. Consequently fabric panels, including the stenciled mesh, must remain mounted on their screen printing frames, known as the printing screen, between print runs. Removal of the mesh and accurate reinstallation for later use represents an improvement over the prior art. Applicant's U.S. Pat. No. 6,739,079 and U.S. Pat. No. 7,536,951 addressed some of the aforesaid problems and this application represents a novel improvement to those teachings and the screen printing industry, and in particular, the fabric panel construction.